1. Field of the Invention
This invention relates to contacting a hydrocarbon-containing gas stream with a physical solvent and particularly relates to contacting a hydrocarbon gas stream with a preferential physical solvent. It more specifically relates to separating and recovering ethane and higher boiling hydrocarbons from a hydrocarbon-containing gas stream and especially relates to simplification of the Mehra Process by elimination of the flashing step. It further relates to specific preferential physical solvents for extractive stripping of a hydrocarbon gas stream.
2. Review of the Prior Art
Hydrocarbons must often be recovered from such gas streams as hydrocarbon gas, alkylates, reformates, and the like. Many recovery processes are available, but countercurrently contacting the upwardly flowing gas stream with a downwardly flowing liquid under conditions furnishing high interfacial surface area is often a preferred recovery process, known as absorption or extraction.
Most physical solvents show some preference among hydrocarbons in a mixture thereof. In other words, they have greater solvency, perhaps because of a stronger physical attraction, for one or more hydrocarbons in such a mixture. This preference is measured by the absorption principle, leading to an alpha or relative volatility. Most of the commonly used lean oils, for example, have relative volatilities of methane over ethane of slightly less than 5.
Lean oils have been used in absorption plants for extracting C.sub.4 + hydrocarbons, with some recovery of propane, from hydrocarbon gas streams for many years. The lean oils are non-selective for lighter hydrocarbons, such as ethane and propane, so that relatively large amounts of methane are absorbed, thereby making the separation of ethane and propane from methane quite difficult and expensive. Due to the market demand for lighter hydrocarbons, such as ethane and propane, and the lack of selectivity of lean oils for such components, the absorption processes have been replaced by processes consisting of refrigerated oil absorption, simple refrigeration, cascaded refrigeration, Joule-Thompson, or cryogenic expander processes. The related Mehra Process as disclosed in U.S. Pat. Nos. 4,421,535, 4,511,381, 4,526,594, 4,578,094, 4,601,738, 4,623,371, 4,617,038, and 4,623,371, is directed toward physical solvents having strongly preferential characteristics. The Mehra Process thereby overcomes the disadvantages of non-selectivity of common lean oils for lighter hydrocarbons, such as ethane and propane.
Furthermore, the recovery levels of various hydrocarbons from the above processes used for the extraction of C.sub.2 + hydrocarbons are quite inflexible. The Mehra Process overcomes the inflexibility drawback by effectively utilizing the selectivity characteristics of preferential physical solvents. Typical recoveries for these processes are compared in Table I.
TABLE I ______________________________________ COMPARISON OF TYPICAL LIQUID RECOVERIES ETH- PRO- BU- GAS- ANE PANE TANES OLINE EXTRACTION (%) (%) (%) (%) ______________________________________ ABSORPTION 4 24 75 87 REFRIGERATED 15 65 90 95 ABSORPTION SIMPLE REFRIG- 35 80 93 97 ERATION CASCADED 70 96 99 100 REFRIGERATION JOULE-THOMPSON 75 96 99 100 EXPANSION TURBO-EXPANDER 85 97 100 100 MEHRA PROCESS 2-98 2-99 2-100 100 ______________________________________
In summary, the oil absorption, refrigerated oil absorption, simple refrigeration, and cascaded refrigeration processes operate at the pipeline pressures, without letting down the gas pressure, but the recovery of desirable liquids (ethane plus heavier components) is poor, with the exception of the cascaded refrigeration process which has extremely high operating costs but achieves good ethane and propane recoveries. The Joule-Thompson and cryogenic expander processes achieve high ethane recoveries by letting down the pressure of the entire inlet gas, which is primarily methane (typically 80-85%), but recompression of most of the inlet gas is quite expensive. The Mehra Process combines the advantages of the higher-pressure extraction processes by selectively recovering and letting down the pressure of essentially the desired components, thereby reducing the compression of undesirable components, such as methane, while achieving high levels of component recovery in a flexible manner.
In all of the above processes, except the Mehra Process, the ethane plus heavier components are recovered in a specific configuration determined by their composition in the raw hydrocarbon gas stream and equilibrium at the key operating conditions of pressure and temperature within the process.
Under poor economic conditions when the ethane price as petrochemical feedstock is less than its equivalent fuel price and when the propane price for feedstock usage is attractive, the operator of a hydrocarbon gas liquid extraction plant is limited as to operating choice because he is unable to minimize ethane recovery and maximize propane recovery in response to market conditions.
The refrigeration process, which typically recovers 80% of the propane, also typically requires the recovery of 35% of the ethane. In order to boost propane recovery to the 95+% level, cascaded refrigeration, Joule-Thompson, or cryogenic turbo-expander processes would have to be used while simultaneously boosting the ethane recovery to 70+% at a considerably larger capital investment.
The parent patents and applications related to the Mehra Process have utilized preferential physical solvents for recovering hydrocarbon gas liquids from hydrocarbon gas streams by extracting the hydrocarbon gas streams with a preferential physical solvent, flashing the rich solvent, and compressing, cooling, and condensing the desired C.sub.2 + hydrocarbons. In carrying out the extraction of desired hydrocarbons according to the extractive flashing version of the Mehra Process, several streams had to be recycled, thereby requiring accessory equipment, such as compressors, coolers, condensers, associated piping, automatic control valves, pressure gauges, and data recording equipment. Furthermore, flashing of the rich solvent stream was carried out in multiple steps, consistent with economic criteria involving energy consumption and capital investment. Even though the energy consumption was lower for the Mehra Process than for conventional state-of-the-art processes, several steps were required that increased the complexity and overall capital requirements of such a plant. There is consequently a need for simplification of the Mehra Process in order to reduce its capital investment requirement.
The Mehra Process in its preferred mode, also utilizes a mixture of dialkyl ethers of polyalkylene glycol, having a molecular weight of 146 to 476 and containing 3-10 ethylene units, for example. While such compounds are satisfactory for the extractive flashing embodiment of the Mehra Process, they are subject to possible further polymerization and/or thermal degradation if cyclically flowing through a unit operation requiring heating, such as distillation, at process temperatures used for separation of mixtures into useful fractions or components. There is, therefore, a need for other solvents that are not subject to these limitations.
U.S. Pat. No. 2,433,286 is directed to extractive distillation of liquid hydrocarbon mixtures with paraffin hydrocarbons as the extractive solvent in a first extractive distillation to produce olefins plus diolefins in the rich solvent and in a second extractive distillation with unsaturated or aromatic hydrocarbons as the solvent at a higher temperature to produce olefins as the raffinate and diolefins in the rich solvent. Paraffins are distilled from the rich solvent of the first extractive distillation and diolefins are distilled from the rich solvent of the second extractive distillation.
U.S. Pat. No. 2,455,803 describes a process for extractive distillation of a vaporizable organic mixture with a solvent comprising a selective solvent and a mutual solvent for the selective solvent and the mixture in order to maintain a single liquid phase. The presence of the solvents in the mixture must cause a greater change in the "escaping tendency" of one component of the mixture relative to that of the other components, "escaping tendency" being defined as the potential of one component to pass from one phase to another. Solvents such as furfural and phenol are named as those having preferential solvent power for aromatic over paraffinic hydrocarbons. Suitable mutual solvents are identified as methyl ketone, cyclohexanone, lactonitrile, morpholine, and aromatic hydrocarbons such as benzene, toluene, cumene, mesitylene, and the like.
U.S. Pat. No. 2,559,519 relates to fractionating a liquid mixture of close-boiling oxygenated compounds in the presence of a large excess of a glycol-ether by continuous fractional distillation in a column of practical size, including a primary rectification zone, a secondary rectification zone above the primary zone, and a stripping zone below the primary zone for countercurrent vapor liquid contact under reboiling and refluxing conditions.
U.S. Pat. No. 2,570,066 is directed to a method of segregating pure hydrocarbons from hydrocarbon mixtures by distractive distillation in the presence of an aromatic hydrocarbon solvent which is preferably a mono-cyclic aromatic hydrocarbon fraction boiling in the range between 365.degree. and 750.degree. F. Mono-cyclic aromatic hydrocarbons having 10 carbon atoms, exemplified by tetramethylbenzenes such as 1,2,4,5-tetramethylbenzene, 1,2,3,5-tetramethylbenzene, and 1,2,3,4-tetramethylbenzene, and further exemplified by 1,2,-dimethyl-3-ethylbenzene, 1,2-dimethyl-4-ethylbenzene, and the like, are preferred. Durene, isodurene, prehnitene, and mixtures thereof are especially beneficial. The ratios of solvent to feedstock may range from about 1:1 to about 20:1, about 5:1 being preferred.
U.S. Pat. No. 2,961,383 is concerned with recovering toluene and xylenes from liquid hydrocarbon mixtures, such as reformed naphthas derived from petroleum, by utilizing certain alkyl phenols as a solvent in extractive distillation. Alkyl phenols or mixtures thereof boiling above 392.degree. F. and having at least 8 carbon atoms are preferred, suitable solvents being xylenols including 2,4-dimethyl phenol, 2,5-dimethyl phenol, ethyl phenols, and trimethyl phenols such as 2,4,6-trimethyl phenol (mesitol) and especially 2,4,5-trimethyl phenol which has a boiling point of 455.degree. F. Cumenol (para isopropyl phenol) is also satisfactory.
U.S. Pat. No. 3,337,425 relates to the recovery of olefin oxide having 3-18 carbon atoms from crude liquid mixtures thereof with oxygenated compounds boiling within 5.degree. C. of the olefin oxides by extractive distillation using, as the extractive solvents, olefinic, naphthenic, and/or aromatic hydrocarbons and having boiling points of at least 35.degree. C. above those of the oxygenated compounds. Representative aromatic compounds suitable as extractive solvents include benzene and alkylbenzenes of 1-9 carbon atoms in the alkyl groups, such as toluene, xylenes, propylbenzene, pseudocumene, mesitylene, durene, and the like.
U.S. Pat. No. 3,445,537 discloses an extractive distillation process for simultaneously obtaining different aromatics with different boiling points from a feed stock containing both aromatics and non-aromatics by contacting the feed stock in at least two extraction-distillation columns of sequentially reduced pressure with an extraction agent and up to 20% by volume of a C.sub.9 aromatic compound. The extraction agent may be propylene carbonate. A mixture of aromatics and extraction agent is withdrawn from the bottoms of the last column and is fed to a plurality of stripper columns. A selected aromatic compound is withdrawn from the top of each stripper column.
U.S. Pat. No. 3,616,271 teaches an extractive distillation method of separating chloroform and/or ethyl acetate from vinyl acetate by using a hydrocarbon having a boiling point of 100.degree.-250.degree. C. as the extractive solvent. Alpha values, as the ratios of relative volatilities determined from equilibrium distillation data for 1% solutions of chloroform and ethyl acetate in vinyl acetate, were calculated and used for evaluating the solvent. The greater the alpha value, the more volatile are liquids being removed as a substantially pure stream from the top of the column while the less volatile liquids are separated together with the extraction solvent from the bottom of the column. Among suitable solvents are alkyl aromatic hydrocarbons such as xylene, triethyl benzene, n-butyl benzene, and mesitylene.
U.S. Pat. No. 4,035,167 relates to recovering ethane and ethylene from methane or from a gaseous mixture, such as that produced in the gasification of coal under pressure. In such a reaction mixture, the combined amount of ethane and ethylene is in the range of 0.05-15 volume % and usually 0.1-3.0 volume %. Selective solvents for absorbing these hydrocarbons include cyclohexane, cyclohexene, 1-chlorobutane, and 1,1,1-trichloroethane, using 0.1-1 liter of liquid solvent per mole of total gas after sweetening thereof. Methanol and n-methylpyrrolidone are inferior solvents.